1. Field of the Invention
The present invention relates to a laser welding method and a laser welding apparatus. More specifically, the present invention relates to a laser welding method and a laser welding apparatus capable of accurately projecting a laser beam on a previously detected weld position.
2. Description of the Related Art
Butt welding using a laser beam is used prevalently for welding members in order to manufacture automobiles, etc. Recently, CO2 lasers have been replaced by YAG lasers capable of focusing a laser beam in a spot smaller than that formable by CO2 lasers. A welding process using a YAG laser capable of focusing a laser beam in a small spot is able to achieve fine welding using a small laser-beam spot. Incidentally, fine welding requires forming the laser-beam spot at a groove in a positional accuracy of 0.1 mm or below.
However, the tracking accuracy of a conventional industrial robot is not so high with a result that the above-mentioned requirement cannot be satisfied. Therefore, a very expensive numerically controlled (NC) welding machine of which price is as high as several tens million Yens is used generally for butt welding using a YAG laser. The necessity of such an expensive NC welding machine is an impediment to the prevalent use of butt welding employing a YAG laser.
This impediment can be removed if a weld line can be tracked by a spot of a laser beam in a high tracking accuracy as mentioned above by an inexpensive industrial robot of which price is, for example, on the order of several million Yens. An industrial robot capable of operating in a high tracking accuracy is described in, for example, JP-A 246660/1994.
This industrial robot detects a weld position by a sensor prior to actually executing a welding operation, and stores data of the weld position obtained by the sensor in a memory. Then, the industrial robot reads the data from the memory when necessary, calculates a correction for correcting the position of the spot of the laser beam on the basis of the data, and executes a tracking control operation so that the laser beam is projected on a desired position.
However, it is difficult for this industrial robot to achieve a highly accurate tracking operation to project the laser beam accurately on a desired position because this industrial robot uses teaching data for calculating the correction for correcting the position on which the laser beam is to be projected.
The present invention has been made in view of such problems. It is therefore an object of the present invention to provide a laser welding method and a laser welding apparatus capable of accurately projecting a laser beam on a weld position without being affected by various errors such as intrinsic to an associated industrial robot.
According to the first aspect of the present invention, a laser welding method of moving a weld portion detector and a laser beam projector disposed behind the weld portion detector by a predetermined distance so that the weld portion detector and the laser beam projector do not move relatively to each other, and projecting a laser beam by the laser beam projector on a portion to be welded detected by the weld portion detector, comprising: sequentially storing pieces of data on weld positions detected by the weld portion detector in combination with times when the pieces of data on the weld positions are obtained and moving speeds at which the weld portion detector is moved in a memory; calculating a time when the weld portion detector passed a point at which the laser beam projector has just arrived on the basis of the times and the moving speeds stored in the memory; and projecting a laser beam by the laser beam projector on the weld position which was detected by the weld portion detector at the time calculated by the calculating step.
The time when the weld portion detector passed a position at which the laser beam projector has just arrived may be determined by calculating back in time a moving distance of the weld portion detector on the basis of the times and the moving speeds stored in the memory and determining the time as a time when the moving distance of the weld portion detector coincides with or exceeds the predetermined distance between the laser beam projector and the weld portion detector. Preferably, the data used for a calculation is erased upon a completion of the calculation of the time when the weld portion detector passed the position at which the laser beam projector has just arrived.
Preferably, the weld portion detector projects a plurality of flat laser beams toward the portion to be welded, images of the portion to be welded irradiated with the flat laser beams are formed by an imaging device, the images are added up to obtain a composite image, and the weld position is detected on the basis of the composite image.
Preferably, a pressing device whose relative position with respect to the laser beam projector is fixed applies pressure to the portion to be welded during a laser welding operation.
Preferably, the laser beam projector projects a YAG laser beam.
According to the second aspect of the present invention, a laser welding apparatus comprising: a weld portion detector of detecting a portion to be welded; a laser beam projector of projecting a laser beam on the portion to be welded; a holding-turning mechanism of holding the weld portion detector and the laser beam projector so as to maintain a predetermined distance between the weld portion detector and the laser beam projector and turning the laser beam projector in a direction perpendicular to a moving direction of the laser beam projector; a moving unit of moving the holding-turning mechanism; and a control unit of controlling a laser welding operation, the control unit sequentially storing pieces of data on weld positions detected by the weld portion detector together with times when the pieces of data on the weld positions are detected and moving speeds at which the holding-turning mechanism is moved in a memory, calculating a time when the weld portion detector passed a position at which the laser beam projector has just arrived, and controlling the laser beam projector and the holding-turning mechanism such that a laser beam is projected on a weld position which was detected by the weld portion detector at the time calculated by the controlling unit.
The time when the weld portion detector passed a position at which the laser beam projector has just arrived may be determined by calculating back in time a moving distance of the weld portion detector on the basis of the times and the moving speeds stored in the memory and determining the time as a time when the moving distance of the weld portion detector coincides with or exceeds the predetermined distance between the laser beam projector and the weld portion detector.
Preferably, the weld portion detector includes a flat laser beam projecting device and an imaging device, the control unit includes an image processing device, the flat laser beam projecting device projects a plurality of flat laser beams arranged at predetermined intervals, the imaging device forms images of parts irradiated with the flat laser beams, and the image processing device adds up the images formed by the imaging device to form a composite image, and a weld position is detected on the basis of the composite image.
Preferably, the laser welding apparatus further comprises a pressing device whose relative position with respect to the laser beam projector is fixed. The pressing device applies pressure to the portion to be welded during the laser welding operation.
Preferably, the laser beam projector comprises a YAG laser.
Preferably, the moving unit comprises a robot.
In the present invention having the above-mentioned features, even if the moving speed of the laser beam projector varies, since the weld position is detected by the weld portion detector disposed a predetermined distance in front of the laser beam projector, the laser beam projector is able to project a laser beam accurately on the weld position detected by the weld portion detector.